1. Field of the Invention
The present invention relates to cleaning of solid substrates, and in particular to compositions and methods for removing hydrocarbon or fluorocarbon residues from such substrates.
2. Description of the Related Art
The persistence of residual oils and greases on a wide variety of manufactured items, particularly articles machined from metal, has long proven problematic. If left on the article, such residues can impose difficulties ranging from inconvenience in handling to measurable performance degradation. The latter is particularly common in high-precision applications where even slight deviations in surface characteristics or frictional properties can seriously interfere with component operation.
Traditional approaches to removal of oils and greases involve dissolution. The soiled article is immersed in a liquid having high solvency power with respect to the adsorbed material, which is thereby removed from the surface. However, if the substrate is not exposed to the solvent for long periods of time, at least some adsorbed surface residue will remain due to mass-transfer limitations and powerful valence forces between the surface and the innermost soil layers. This limitation is exacerbated as the solvent becomes saturated with soil. The need for strong solvency action has also led to the use of chemicals that are environmentally harmful (e.g., chlorofluorocarbons), toxic (e.g., chlorocarbons), and/or dangerously flammable (e.g., alkanes).
As an alternative, industry has adopted cleaning techniques utilizing various types of surface-active agents, or surfactants, which facilitate the use of environmentally benign solvents that ordinarily would not act on typical soils. The molecular structures of surfactants are amphiphilic, containing a solvent-soluble (usually polar) lyophilic part and a lyophobic part (usually nonpolar) compatible with soils but insoluble in the solvent. The strong interaction between the polar molecular region and the polar solvent is exploited to "drag" soils into micelle structures, which disperse throughout the solvent and away from the substrate.
Unfortunately, traditional surfactant-based cleaning mixtures exhibit certain limitations. Most are aqueous solutions, and not all substrates are compatible with water; for example, metal items can corrode, and electronic equipment can be destroyed if immersed in water-based cleaning media. Furthermore, water, with its high heat of evaporation, requires significant time to air dry. More volatile solvents offer dry-to-dry cleaning cycles under ambient or modestly elevated temperatures; such solvents, however, may not be compatible with readily available surfactants. Finally, water can be difficult or expensive to obtain at the high purity levels needed for precision cleaning applications.